1. Technical Field
The present disclosure relates to blending certain polyhedral oligomeric silsesquioxanes (POSS) or polyhedral oligomeric silicates (POS) with ethylene-based polymers. The addition of POSS and/or POS reduces flammability, improves oxidation resistance, increases permeability to gases, and improves heat distortion temperature and mechanical strength of the polymers.
2. Background of Related Art
Ethylene polymers are well known in the art. In general, polyethylenes are divided into the following groups: High Pressure, Low Density Polyethylenes (LDPE); Linear Low and Medium Density Polyethylenes (LLDPE); High Density Polyethylenes (HDPE); Ultrahigh Molecular Weight Polyethylenes; and Modified Polyethylenes.
The physical state of a polymer is dependent upon its temperature. At low temperatures, polymers are physically stiff and act like glass. Upon heating to a temperature within a range known as the glass transition region, polymers soften and behave in a leathery or rubbery manner. The glass transition temperature, Tg, the temperature at which a polymer changes from glassy to rubbery behavior, is an important property used to characterize a polymer. Qualitatively, the glass transition temperature can be interpreted as the temperature at which the chain segments of a polymer exhibit long-range, coordinated molecular motion. In a physically or chemically crosslinked system, the number of chain segments involved in coordinated molecular motion is reduced, which results in an increase in Tg.
The Tg of polymers can be observed experimentally by measuring various thermodynamic, physical, or mechanical variables as a function of temperature. The most direct determination of Tg involves measuring the effects of temperature changes on Young""s modulus, a fundamental measure of the stiffness of a material when stretched. Young""s modulus is defined as follows:
E="sgr"/xcex5
where "sgr" and xcex5 represent the tensile stress and strain, respectively. The higher the value of Young""s modulus, the more resistant the material is to deformation.
The use of dynamic mechanical analysis (DMA) to study the effects of temperature on the stiffness of polymers is well known to those skilled in the art of polymer (or copolymer) characterization. In DMA, Young""s modulus has a more complex definition:
E=Exe2x80x2+iExe2x80x3
where Exe2x80x2 is the storage modulus and Exe2x80x3 is the loss modulus. The storage modulus is a measure of the energy stored elastically during deformation, and the loss modulus is a measure of the energy converted to heat.
The loss factor, or loss tangent, is another equation widely used by those skilled in the art of polymer (or copolymer) characterization to determine the value of Tg. The loss tangent, tan xcex4, is the ratio of loss modulus to storage modulus and is defined as follows:
tan xcex4=Exe2x80x3/Exe2x80x2
In DMA, the maximum value of Exe2x80x3 or tan xcex4 during a temperature scan can be used to determine the value of Tg.
Instead of stretching, the molten polymer may be subjected to shearing deformation to determine its viscosity. The viscosity of a polymer melt is strongly related to the molecular properties of the polymer including, but not limited to, molecular weight, molecular weight distribution, and crosslinking. Due to the limited mobility of polymer chains, a physically and/or chemically crosslinked polymer system has a higher viscosity than a non-crosslinked polymer system. In rheological studies of polymer melts the shear stress, f, is closely related to the viscosity, xcex7, by the following equation:
f=xcex7(ds/dt) 
where f and s represent the shear stress and strain, respectively, and t is time. A polymer melt that is physically and/or chemically crosslinked is sometimes referred to a gel sample; in such a case, the shear stress is referred to as xe2x80x9cgel strengthxe2x80x9d and a higher shear stress at the same shear rate indicates a higher gel strength.
Polyhedral oligomeric silsesquioxane or polyhedral oligosilsesquioxane (POSS) cage molecules and polyhedral oligomeric silicate (POS) (spherosilicate) cage molecules or reagents are organic silicon compounds that are increasingly being utilized as building blocks for the preparation of novel catalytic materials and as performance enhancement additives for commodity and engineering polymers. The physical sizes and structures of POSS and POS reagents are on the nanometer dimension (10xe2x88x929 m). Accordingly, POSS and POS reagents are frequently described as the smallest xe2x80x9csilica-likexe2x80x9d particles possible. Their nanometer size and unique hybrid (inorganic-organic) chemical composition are responsible for the many desirable property enhancements which have been observed upon incorporation of POSS/POS reagents into polymer systems. POSS and POS exhibit a number of potentially useful properties including high temperature stability in air and good adhesion to a number of substrates. POSS and POS are also resistant to oxidation and degradation by ultraviolet light.
The preparation of functionalized POSS monomers and their use in the synthesis of polymers has been previously disclosed. For example, U.S. Pat. No. 5,484,867 discloses a process for the preparation of reactive POSS monomers which can be chemically reacted with oligomers, polymers, catalysts or co-monomers to form polyhedral silsesquioxane polymers containing silsesquioxanes as pendant, block, or end group segments. As another example, U.S. Pat. No. 5,939,576 discloses a process for the preparation of reactive POSS and POS (spherosilicate) by metal catalyzed hydrosilylation reactions of silane containing POSS or POS with olefinic reagents bearing functionalities useful for grafting reactions, polymerization chemistry and sol-gel process. The functionalized POSS or POS monomers prepared by the above two patents are used to prepare polymer systems wherein POSS or POS is chemically linked to a polymer.
The chemical copolymerization of these functionalized POSS or POS monomers with other oligomers, polymers, catalysts or co-monomers has several drawbacks. First, POSS and POS exhibit a well-known propensity to form insoluble, intractable gels. In addition, both the functionalization and the copolymerization processes are expensive and difficult to control. Further, the resulting chemical copolymers, because of their highly crosslinked nature, have been difficult to handle, purify and characterize. The polymer quality and utility is limited due to impurities which arise from side reactions during synthesis and the resulting polymers have a limited shelf life because they contain reactive functionalities. Also, the copolymers have increased viscosity which, in many cases, is undesirable.
Accordingly, the need still exists for ethylene-based copolymers which are essentially free of impurities and whose properties may be enhanced and controlled by the incorporation of POSS and/or POS in the polymer.
The present invention meets these needs by providing compositions of ethylene-based polymers physically blended with certain hydrocarbon-substituted POSS and/or POS molecules. These blends have been found to exhibit enhanced properties, including, for example, elastometric properties, reduced flammability, increased glass transition temperature, increased permeability to gases, and improved UV stability, oxidation resistance, heat distortion temperature and/or mechanical strength.
The present compositions contain at least one ethylene-based polymer blended with a POSS and/or a POS having hydrocarbon substitution.
When present, the POSS utilized in the present blend compositions has the general formula [RSiO1.5]n where n is an even number and R is selected from the group consisting of substituted or unsubstituted aliphatic or aromatic hydrocarbon groups having one to thirty carbon atoms. When POS is utilized in the present blends, the POS has the general formula [RMe2SiOSiO1.5]n where n is an even number and R is selected from the group consisting of substituted or unsubstituted aliphatic or aromatic hydrocarbon groups having one to thirty carbon atoms.
The POSS and/or POS is present in the blend compositions in an amount up to about 30% by weight of the blend. In a preferred embodiment, the POSS and/or POS is 1 to about 20% by weight of the blend. It is most preferred to have POSS and/or POS present in an amount from about 2 to about 10% by weight of the blend.
Methods for making compositions containing at least one ethylene-based polymer and at least one hydrocarbon-substituted POSS and/or POS are also described. The blending or mixing process can be carried out by melt blending or solution blending the polymer(s) and cage molecule(s). In a preferred embodiment, the blend is obtained by physically combining the ethylene-based polymer with the POSS and/or POS to form a mixture and then heating the mixture to a temperature above the glass transition temperature of the polymer while stirring.
Without wishing to be bound by any theory, it is believed that the dispersed POSS or POS molecules behave as physical crosslinkers in the matrix of ethylene-based polymers to which they are added and impart enhanced characteristics without undesirable characteristics that can result from chemical cross-linking.